1. Field of the Invention
This invention relates generally to shape memory alloys (SMAs), and more particularly to a method for post-production precision modification and enhancement of pre-existing shape memory alloy (SMA) forms to precisely alter their chemical composition and properties and produce greater accuracy and versatility. The process can also be used to impart shape memory properties to non-SMA alloy forms.
2. Brief Description of the Prior Art
Shape memory alloy (SMA) materials undergo a reversible phase transformation in their crystal structure when heated from a low temperature form to a high temperature form. Transformation temperatures can be accurately set between -200.degree. C. to 200.degree. C. by varying the composition of the alloy and annealing procedure when forming the shape memory alloy wire. Suitable shape memory alloy materials include, but are not limited to: Ag--Cd, Au--Cd, Cu--Al--Ni, Cu--Sn, In--Ti, Ni--Al, Ni--Ti, Ni--Te--Fe, Ni--Ti--V, Ni--Ti--Al, Fe--Mn--Si, Cu--Zn--Al, U--Ni, U--Mo and U--Nb.
The terms used herein are meant to have the following meanings. The "austenitic start temperature" (A.sub.s temperature) is the temperature at which a shape memory alloy starts transforming to austenite upon heating. The "austenitic finish temperature" (A.sub.f temperature) is the temperature at which a shape memory alloy finishes transforming to austenite upon heating. "Austenite" is the higher temperature phase present in Ni--Ti, for example.
The "martensitic start temperature" (M.sub.s temperature) is the temperature at which a shape memory alloy starts transforming to martensite upon cooling. The "martensitic finish temperature" (M.sub.f temperature) is the temperature at which a shape memory alloy finishes transforming to martensite upon cooling. "Martensite" is the more deformable, lower temperature phase present in Ni--Ti, for example. "Hysteresis" is the temperature difference between a phase transformation upon heating and cooling. "Shape memory" is the ability of certain alloys to return to a predetermined shape upon heating via a phase transformation. "Superelasticity" is the springy, "elastic" behavior present in shape memory alloys, such as Ni--Ti, at temperatures just above the A.sub.f temperature. The superelasticity arises from the formation and reversion of stress-induced martensite.
Upon heating or cooling, shape memory alloys do not completely undergo their phase transformation at one particular temperature. Instead, the transformation begins at one temperature (known as the start temperature) and is completed at another temperature (known as the finish temperature). Further, there is a difference in the transformation temperatures upon heating from the first phase to the second phase (martensite to austenite for example in Ni--Ti) and cooling from the second phase to the first (austenite to martensite), resulting in a delay or "lag" in the transformation. This difference is known as the transformation temperature hysteresis. The transformation temperature hysteresis can also be effected by alloying, cold working, and heat treatment.
Shape memory alloy (SMA) and the shape memory effect (SME) were initially defined by the NASA report NASA-SP 5110. The shape memory effect (SME) is observed in alloys, which exhibit a thermoelastic martensite transformation. Martensitic transformations are responsible for the hardening that occurs when steel is quenched from an elevated temperature. Unlike martensite in steel, the thermoelastic martensite in shape memory alloys, after being formed by quenching from an elevated temperature, will then appear and disappear upon subsequent heating and cooling over a small temperature range.
Typically, the conventional method of creating the shape memory effect (SME) is to heat a specimen to an elevated temperature followed by a rapid quench. Since the change in crystalline structure is diffusionless, the martensite structure appears spontaneously. If the specimen is deformed within the recoverable strain limit and then heated above its transformation temperature (A.sub.s), an orderly transformation of martensite groups causes the specimen to recover its original unstrained shape (remembered shape). Total recovery of the remembered shape occurs at the A.sub.f temperature (after sufficient heat has been absorbed) which is greater than the A.sub.s temperature. For example, the transformation temperature can be set between -200.degree. C. to 200.degree. C. by varying the composition of the alloy and annealing procedure.
The shape memory properties of Shape Memory Alloys (SMAs) are extremely sensitive to very slight changes in their chemical composition. A change of 1% nickel in NITNOL (Nickel/Titanium SMA) can result in 80.degree. C. difference, or more, in the transition temperatures (A.sub.s or A.sub.f or M.sub.s or M.sub.f) of the alloy.
Common metallurgical techniques are limited to the accuracy of measurement, purity of the raw materials, and the nature of preparation method. If the industry had to rely solely on the conventional prior art production techniques, the obtainable accuracy of the austenitic start temperature A.sub.s and austenitic finish temperature A.sub.f could be as poor as plus or minus 15.degree. C. for significant quantities of SMA.
Conventional augmentary techniques such as cold working and adjusting the conditions of the memory imparting process have been used to adjust the transition temperatures to a finer degree but are limited and complex.
There are several patents that disclose various methods for producing shape memory alloys having various physical properties.
Goldstein et al, U.S. Pat. No. 4,283,233 discloses a method of changing the shape change transition temperature range (TTR) of an object made from a nickel-titanium based shape change memory alloy by selection of the final annealing temperature. In this method pre-existing shape memory alloy powder is blended with other alloy powders or elemental powders, compacted, and diffused to form SMA having a predetermined transition temperature range.
Fountain et al, U.S. Pat. No. 4,310,354 discloses a process for producing a shape memory effect alloy having a desired transition temperature utilizing at least one prealloyed shape memory effect alloy powder having a chemistry similar to that of the to be produced alloy and a transition temperature below the desired transition temperature of the to be produced alloy; and at least one other prealloyed shape memory effect alloy powder having a chemistry similar to that of the to be produced alloy and a transition temperature in excess of the desired transition temperature of the to be produced alloy. The prealloyed powders are blended, consolidated, and thermally diffused to provide a substantially homogeneous alloy of the desired transition temperature.
In finished TiNi SMA forms made with the powder metallurgy method wherein Ti powder and Ni powder are mixed at suitable range and are sintered by heat treating diffusion; because the powder has a large surface area and the oxide layer formed at the surface of the Ti powder (which is apt to oxidize), the finished product can have voids which may cause variations in transformation temperature, and the diminution of strength and life.
Ishibe, U.S. Pat. No. 4,830,262 discloses a method of making titanium-nickel alloys (TiNi) having a homogeneous composition by consolidation of compound material. The process includes forming a composite by providing in a sheathing container plural pieces of compound wire having Ti lineal wire made of Ti material and Ni material made to contact at least a portion of the surface of the Ti lineal wire. The composite is then subject to dimension-reduction, after which diffusion is effected to cause the production of a TiNi phase. The composite is removed from the sheathing container and cold-worked.
Thoma et al, U.S. Pat. No. 4,881,981 discloses a process for adjusting the physical and mechanical properties during formation of a shape memory alloy member of a known chemical composition by increasing the internal stress level and forming the member to a desired configuration and then heat treating the member at a selected memory imparting temperature.
Ebato et al, U.S. Pat. No. 5,316,599 discloses a method of producing Ni--Ti intermetallic compounds by subjecting a laminate of Ni foils and Ti foils to a rolling for thickness adjustment and then to a diffusion heat treatment at multistages within a particular temperature range for a particular time. In this method the compounds are produced by alternately laminating plural Ni foils and Ti foils one upon the other, rolling the resulting laminate to a final product thickness and then subjecting it to a heat treatment to form a Ni--Ti intermetallic compound having Ni content of 48-55 atomic %.
Shiba et al, U.S. Pat. No. 4,659,437 discloses a method of thermal diffusion alloy plating for steel wire (not shape memory alloy) wherein the quantity of metal plated, plated metal composition, plating composition gradient or combination thereof, is continuously detected by an energy dispersive type X-ray fluorescent analyzer. Upon detecting any variation in these values, a control signal is given to a control unit to automatically adjust the plating electric current and the diffusion heating quantity to impart the desired quantity of metal plated, the desired plated alloy composition ratio, and the desired plating composition gradient uniformly in the lengthwise direction of the steel wire.
The present invention is distinguished over the prior art in general, and these patents in particular by a method for post-production precision modification and enhancement of pre-existing shape memory alloy (SMA) forms to precisely alter their chemical composition and properties and produce greater accuracy and versatility. The present process includes carefully controlled electrodeposition or electroless deposition of alloying agents such as nickel, copper, or other elements on the surface of the pre-existing SMA form to produce the desired shape memory properties, followed by application of a protective coating, then high temperature heat treatment in combination with gentle physical kneading to speed up the homogenization process (thermal diffusion) and produce rapid diffusion of the plated metal throughout the form. The present method may also use the skin effect of AC current for localized heat treatment of the SMA form and neutron activation tracer analysis for in-line quality control and diagnostics. The process can also be used to impart shape memory properties to non-SMA alloy forms. Another aspect of the invention is an article of manufacture produced by the process.
The present method and process offers a significant improvement in the ability to adjust the chemical composition of pre-existing shape memory alloy (SMA) forms (wire, rod, sheet, ribbon etc.), more accurate customization of the shape memory properties of an SMA in small lots, and allows reduction in the types of SMAs having different physical properties that a manufacture would have to maintain in inventory.